Scary thought. I have a number of major textbooks I use on a regular basis on my Mac. All of them html format, from publishers websites direct for use off line when I am in places where the internet just doesn't go. I own the textbooks and have the right to use them because of this and the fair use doctrine.
I can prove I own the works by producing the original textbooks, no problem. But I did it so I wouldn't have to carry around a dozen multi-volume thousands of pages textbooks which would probably exceed the 45 kg luggage limit. But I can't prove it on the road. This definitely meets the fair use exemption.
I think LuvsParis is right: proving you own it at the border when they're snooping in your private correspondence may be impractical, but the place for proofs is in court, not at the border. I did check with my shyster er patent guy. His comment: Fair use includes changing the media from one to another, so long as it doesn't extend the use beyond what the original media would have permitted. He's in the DC circuit, but practices internationally.